1. Field of the Invention
The present invention relates to a traction control device for a vehicle.
2. Description of the Related Art
In a known traction control device, to suppress the occurrence of slippage of the driven wheel when the engine is accelerated, an additional subthrottle valve, which is not connected to the accelerator pedal, is arranged in the intake passage of the engine as a supplement to the main throttle valve connected to the accelerator pedal. The closing operation of the subthrottle valve is controlled in accordance with the amount of slippage, to reduce the output torque of the engine (see, for example, Japanese Unexamined Patent Publication No. 63-109247).
Further, an engine is normally equipped with an exhaust gas recirculation (hereinafter referred to as an EGR) device for recirculating the exhaust gas into the intake passage of the engine, to reduce the amount of NO.sub.x in the exhaust gas. This EGR device is controlled in response to the level of vacuum acting on the vacuum port formed in the inner wall of the intake passage. This vacuum port is normally positioned near the throttle valve, so that the vacuum port is open to the intake passage upstream of the throttle valve when the degree of opening of the throttle valve is smaller than a fixed degree, i.e., when the engine is operating under a light load, and that the vacuum port is open to the intake passage downstream of the throttle valve when the degree of the throttle valve exceeds the fixed degree. In this case, when the engine is operating under a light load, a vacuum does not substantially act on the vacuum port, and at this time, the recirculating operation of the EGR gas is stopped. When the engine is operating under a medium load, i.e., when the throttle valve is half open, since the vacuum port is opened to the intake passage downstream of the throttle valve, a large vacuum acts on the vacuum port, and at this time the EGR gas is recirculated. Conversely, when the engine is operating under a heavy load, i.e., when the throttle valve is fully open, although the vacuum port is open to the intake passage downstream of the throttle valve, a vacuum does not substantially act on the vacuum port, and accordingly, the recirculating operation of the EGR gas is also stopped at this time.
Furthermore, an engine is also normally equipped with a charcoal canister for temporarily adsorbing fuel vapor generated in the fuel tank, to prevent the fuel vapor from escaping to the outside air. This charcoal canister is normally connected to a purge port formed on the inner wall of the intake passage, and when a vacuum acts on the purge port, the fuel vapor adsorbed by the charcoal canister is purged into the intake passage from the purge port, and the thus-purged fuel vapor is burned in the engine cylinders. This purge port is normally arranged near the throttle valve so that the purge port is open to the intake passage upstream of the throttle valve when the degree of opening of the throttle valve is smaller than a fixed degree, i.e., when the engine is operating under a light load, and that the purge port is open to the intake passage downstream of the throttle valve when the degree of the throttle valve exceeds the fixed degree. In this case, when the engine is operating under a light load, a vacuum does not substantially act on the vacuum port, and at this time, the purging operation of the fuel vapor is stopped. When the engine is operating under a medium load, i.e., when the throttle valve is half open, since the purge port is opened to the intake passage downstream of the throttle valve, a large vacuum acts on the purge port, and at this time, the fuel vapor is purged. Conversely, when the engine is operating under a heavy load, i.e., when the throttle valve is fully open, although the purge port is open to the intake passage downstream of the throttle valve, a vacuum does not substantially act on the purge port, and accordingly, the purging operation of the fuel vapor is stopped at this time.
Where such an EGR device is incorporated into an engine equipped with such a traction control device, the vacuum port of the EGR device is arranged near the main throttle valve, so that the level of vacuum acting on the vacuum port can be controlled by the main throttle valve, and the EGR gas is fed into the intake passage downstream of the main throttle valve. In this case, since a part of the EGR gas reaches the main throttle valve, carbon, etc. contained in the EGR gas adheres to the main throttle shaft. Nevertheless, even if carbon, etc. is adhered to the main throttle shaft, since the main throttle valve is operated by the accelerator pedal under a strong power, there is no danger that the main throttle valve can not be controlled. Nevertheless, the subthrottle valve normally remains fully open, and the subthrottle valve is controlled by an electric motor under a weak power. Consequently, if carbon, etc. is adhered to the subthrottle valve shaft, there is a danger that the subthrottle valve will not be able to operate. Consequently, to prevent the EGR gas from reaching the subthrottle valve, the subthrottle valve is normally arranged upstream of the main throttle valve.
Nevertheless, if the vacuum port of the EGR device is arranged near the main throttle valve, and the subthrottle valve is arranged upstream of the main throttle valve, the following problem will arise. Namely, when the engine is operating under, for example, a heavy load, since a vacuum does not substantially act on the vacuum port of the EGR device as mentioned above, the recirculation of the EGR gas is stopped. At this time, however, if the slippage of the driven wheel occurs, since the subthrottle valve is closed, a large vacuum acts on the vacuum port of the EGR device, and thus the recirculation of the EGR gas is carried out. Accordingly, since the subthrottle valve is closed to a position in which a reduction in the output torque of the engine, which reduction suitably suppresses the slippage of the driven wheel, can be obtained while the recirculation of the EGR gas is not carried out, if the recirculation of the EGR gas is carried out at this time, as mentioned above, the output torque of the engine is considerably reduced. As a result, problems arise in that surging or misfiring of the engine will occur, and there is a danger of an engine stalling.
Further, where the purge port of the charcoal canister is arranged near the main throttle valve, and the subthrottle valve is arranged upstream of the main throttle valve, if the subthrottle valve is closed when the engine is operating under a heavy load, since a large vacuum acts on the purge port, fuel vapor is fed into the intake passage, and as a result, there is a danger that the air-fuel mixture will become excessively rich and thus engine stalling will occur.
In addition, in a majority of fuel injection type engines, the actual fuel injection time is calculated by multiplying a basis injection time by both the feedback correction coefficient and the learning coefficient. The feedback correction coefficient is controlled by the signal output from the oxygen concentration detector arranged in the exhaust passage of the engine, so that the air-fuel ratio of the air-fuel mixture becomes equal to, for example, the stoichiometric air-fuel ratio, and the learning coefficient is controlled so that the mean value of the feedback correction coefficient is maintained at about 1.0. This learning coefficient is provided for equalizing the air-fuel ratio with the stoichiometric air-fuel ratio when both the recirculation of the EGR gas and the purging operation of the fuel vapor are not carried out, and thus the learning coefficient is renewed in an engine operating state in which both the recirculating operation of the EGR gas and the purging operation of the fuel vapor are to be stopped. Nevertheless, even if the engine is operating in a state in which both the recirculation of the EGR gas and the purging operation of the fuel vapor is to be stopped, when the subthrottle valve is closed, the EGR gas and the fuel vapor are fed into the intake passage, and the air-fuel mixture becomes lean or rich. At this time, since the learning coefficient is renewed, the learning coefficient deviates from a correct value, and thus a problem arises in that the air-fuel ratio will deviate from the stoichiometric air-fuel ratio after the traction control is completed.